Pulse modulation system



8, 1950 L. J. LIBOIS 2,518,341

" PULSE MODULATION SYSTEM Filed May 28, 1949 FIG] Lou/s Z7Z'SEFH L IBO/S2 W m ,4 d,

t inafter by two embodiments shown by way of iatenteci Aug. 8, 1956PULSE MODULATION SYSTEM LouisnJos eph Libois, Paris, France IApplicatio'nMay 28,- 1949,s er 1'N 95,910

- In France Claims.

The present invention relates to an amplitude modulating deviceforelectric pulses.

'In the pulse modulation technique, one often has to modulate 'pulses-in amplitude; either be cause this'modulation is utilized directlyortbecause said modulation constitutes a convenient:

I stage for the production of other types of modulation, induration orin position, for instance.

The amplitude pulse modulation device which is the'object oftheinvention, ofier's, with respect to other-known devices,generallyutilized, a num- 1 her of advant'agesnamely: simplicityand economy,linearity of modulation, stability as regards the- 'variations' of thesupply voltages, the possibility of replacing the modulating electrontube by another one, without the necessity of altering any adjustment,-the possibility of transmitting wide modulation bands without anydifficulty and of using'low values of the low frequency excitation.

of the invention applying particularly to a, multiplex transmissionsystem, the modulating device comprises the-'superpositionon the controlgrid of an electron tube of the low frequency modulat ingvoltage and ofunblocking pulses, the modulating voltage being applied to the primaryof a transformer whose secondary is located between the grid oftheelectron tubeand the point common to a resistance and to a-detectorwhich clips the signals from a pulse generator.

According to anothercliaracteristic and to an arrangement of theinvention applying to very wide modulation bands, a, low frequencytransformer energizes a relatively low resistance placed in series withthe detector. e

l The invention will be described in detail herenonlimitative examples;The description will be given with refer'ence'to the appended drawingswhereinz. Figure 1 shows a device which can be adapted particularly welltomultiplex transmission; Figure 2 shows a modification of the device ofFigure 1, designed especially for the transmission of wide frequencybands.

In Figure 1, I shows a sourceof pulses having the desiredcharacteristics for the type-of trans mission utilized. In the ,caSe ofa multiplex transmission, for instance, the pulse sources may be tapsuniformly distributed on a delay line. Resistance 3 is a resistancesufiiciently large with respect to the internal impedance of detector 4for the peaks of the pulses generated at l, assumed to be imperfectlyrectangular, appearing on the input grid of the electron tube 5 to bewell Characteristically and according to a feature June '10; 1948- (Cl.332 v clipped. The eifect of this clipping action is primarily to limitto. a predetermined value the amplitude of the pulse voltage as suppliedby this generator to the grid of tube 5. It is obvious that thislimitation: point of control. electrode voltage can be made'to lieinside the linear portion of the anode current/control electrode voltagecharacteristic curve of said tube 5. Resistance 6 and condenser I give,on the other hand, 1 abiassing voltage of such a value that theelectrontube 5 is rendered operative only at the time of occurrence of apulse 2. 3

The low frequency modulating voltage'is applied between 8 and 9 to theprimary of a trans- 15,- ;former l0 whose se'condary winding isconnected across a suitable impedance'l i. This transformer is designedso as to offer the least possible amount.

ofstray coupling capacities was to avoid distortion of the pulse appliedto the grid of tube 5. m The, insertion ofthis transformer between thegridof 5 and the point common to 3 and 4 pre-. vents the low frequencyvoltage from reacting on the pulse source, since theimpedance of thegrid circuit is very high atv low frequencies; Thus, one avoids, in thecaseof a multiplex transmission, the cross-talk due to couplings betweenthe various channels through the medium of the pulse source.

Resistance durin gthe passage of the pulse 2; it also contributes, ifnot uncoupled by a condenser, to the improvementof the linearity ofmodulation. [3 V is the load impedance of the tube. l4 shows an outputpulse modulated in amplitude.

In the case of a multiplex transmission, the plates of the modulatingtubes, such as 5 are connectedtogether. If the type of modulationultimately desired in the considered multiplex transmission system isnot that of amplitude modulation, it will be sufficient to eifect laterthe change in type of modulation in elements common to all channels. H

Figure 2 shows a modification, in accordance with the invention, of thecircuit described previously. This modification is of particularinterest in case very wide modulation bands have to be transmitted overa single transmission channel.

I5 is a generator delivering pulses with asuitable recurrence period andwhose duration is large enough with respect tosaid period, a conditionnot troublesome for a single transmission channel. Under suchconditions, if, for instance, the pulse generator does not transmit adirect current, (a transformer or a resistance-capacity Iisuitablybiasses the electron tube coupling are suflicient to thiseffect) the pulses, at the output of [5, have the shape shown in [6, thedotted line H representing ground potential. These pulses are applied tothe detector [9 through the resistance [8. The low frequency modulatingvoltage is applied, between 2| and 22, across resistance 20 in serieswith detector l9. 2| and 22 may be connected to the secondary of a lowfrequency transformer which, in this set-up, does not need to beespecially designed with low stray coupling capacities.

The resistance 20 must be comparatively low with respect to theresistance l8. The signal on the grid of the electron tube 23 is thusthe superposition of the low frequency voltageand of the I pulses.Resistance 24 fixes the bias during the passage of one pulse and,futher, improves the,

linearity of modulation. Since the tube .23 is blocked between twopulses, its grid potential remains sufficiently below. ground potential.An amplitude modulated pulse 26'is obtainedat the terminals of the loadimpedance .25.

The whole of this device doesnot comprise any condenser, any. especiallydesigned transformer nor any time constant and may, if de-.

sired, transmit low frequencies, starting from zero frequency.

The electron tubes, in figures l and 2 have been shown as simpletriodes. It is obviously understod that, if desired, tubes having morethan three:

electrodes, such as pentodes, for instance, can be utilized.

I claim:

rent characteristic curve of the tube and means for applying to saidcontrol electrode a biassing voltage of such value as to reduce theanode current to zero in the absence of the pulse voltage.

6. In a transmission system for recurrent amplitude modulated electricpulses; an electronic tube having a cathode, control electrode andanode; generator of recurrent electric pulses, a transformer to theprimary winding of which the modulating voltage is applied, a circuitconnecting said pulse generator and control electrode and including aresistor and the secondary 1. In a transmission system for recurrentamplitude modulated electric pulses, an electronic tube having anode andcontrolelectrodes, means forapplying to a, .controlelectrode amodulating voltage and a pulse voltage of the recurrence of theaforesaid pulses, means for applying to said control electrode abiassing voltage of suchvalue as to block the. flow of anode current inthe absence of the applied pulse voltage, and

further circuit means to modify the amplitude of tube having a cathode,control electrode and.

the applied pulses to such a degree that in-the absence of modulatingvoltage the voltage of the control electrode due topeaks of the modi-'sing means for said detector causing it to clip the pulses appliedthereto.

5. In a, transmission system for recurrent amplitude modulated electricpulses, an electronic tube having an anode and control electrode, meansfor applying to the control electrode a modulating voltage and a pulsevoltage,'a detector having output means connected to said controlelectrode and means applying the pulse voltage to;

said detector; means for biassing said detector to such a degree that,in the absence of modulating voltage, the voltage of the controlelectrode dueto the peak pulse voltage lies within the linear portion ofthe control electrode-anode cur- Number winding of said transformerconnected in series;

--a, detector having one terminal connected to a point between saidresistor and said secondary winding and its other terminal to a point ofsubstantially constant potential, means for applying to said controlelectrode a bias-sing voltage of such value as to reduce the anodecurrent to zero in' the absence of the pulse zvoltagethe resistance ofsaid resistor being suflicient to limit toa. predetermined peak valuethe amplitude of the pulse voltage applied to the. control electrode,,the voltage of the control electrode remaining within the linearportion of. the control electrode-anode current characteristic curve ofthe tube during the period of peak pulse voltage. ;7. A transmissionsystem according .to claim 6, in which the point of constant potentialto which the. detector terminal is connected, is a. terminal of, aresistorconnected in series in the anodecathode circuit of the tube.

8. A transmission system according to claim 5,

wherein the pulse voltage andthe modulating,

voltage are applied in series to the detector whereby the peak value ofthe pulse voltage varies linearly with the amplitude of themodulatingvoltage. W

9. In a transmission system for recurrent am plitude modulated electricpulses; an electronic anode; a pulse voltage generator, a circuitconnecting said generator and controlelectrode; a detectorzand resistoconnected in series, one terminal of saiddetector being connected tosaid circuit whereby the peak pulse voltage applied to said controlelectrode is limitedto a predetermined value; the resistance of saidresistor being substantially less than the internal resistance of thepulse generator, means for applying the,

modulating voltage to the terminals of said resistor and means forapplying to said control,

electrode a biassing voltage of such ,value as to reduce the anodecurrent to zero in the absence of the pulse voltage.

, 10, In a transmission system according to claim 9, wherein the circuitconnecting the. pulse generator and control electrode includes asecondresistor connected in series, said second resistor having aresistance substantially greater than that of the resistor in serieswith the detector.

LOUIS JOSEPH LIBOIS.

REFERENCES CITED The following references are of record in the file ofthis patent:

FOREIGN PATENTS Country Date 793,162 France Jan. 18, 1936

